For many years, polyethylene coated paper products have been in popular use for the manufacture of milk cartons, paper cups, disposable diapers and the like. Manufacturers of these items generate considerable amounts of scrap material from edge trimmings, stampings, and rejects. This scrap material is routinely reprocessed in conventional hydropulpers to recover the cellulose fiber. The polyethylene, separated from the fiber slurry in the hydropulper by coarse screens, is discarded by dumping in land fill areas or by incineration. It is estimated that from about 25 to 50 million pounds of polyethylene waste, on a dry polymer basis, are generated annually in the United States by these fiber recovery plants.
The polyethylene waste is obtained from the hydropulper in the form of narrow strips or folded pieces of thin film contaminated with cellulose fiber and water, resembling very coarse saurkraut. Typically, the waste material contains approximately 40 to 70% by weight water and 30 to 60% dry solids, i.e., the cellulose fiber and polyethylene with the water removed. Approximately 10 to 40% by weight of the dry solids is cellulose fiber. Thus, for example, a particular batch of waste material having 50% water and 20% cellulose contamination would have an overall composition of 50% water and 50% dry solids of which 20% is cellulose fiber and 80% is polyethylene. The bulk density of the dry solids is about 3 pounds per cubic foot; approximately 10-20 pounds per cubic foot in wet form.
Under current practice, the wet polyethylene waste material is loaded onto trucks and dumped in land fill areas as refuse or mixed with other burnable wastes and incinerated. Such disposal procedures are expensive and undesirable. For land fill disposal, the material is bulky, consumes much space, is unsightly, and is easily blown away by winds. For incineration, the bulky nature of the material creates problems in feeding the material to the incinerators, the high water content reduces combustion efficiency, and the thermoplastic nature of polyethylene results in the material fusing and sticking to the grates. Furthermore, polyethylene is not biodegradable and remains in the land fill areas for very long periods of time, restricting the further reuse of the area. Thus, this material is considered to have a negative value and to be a nuisance in disposal areas.
Many attempts have been made to utilize this polyethylene waste material directly or to reprocess it into a more useable form. However the high water contend and low bulk density of this material has prohibited its economic utilization. Most plastic processing equipment cannot utilize materials of low bulk density, generally being designed with compaction ratios of about 2 or 3 to 1. The polyethylene waste material, for possible utilization, must be compacted from about 3 pounds per cubic foot to about 60 pounds per cubic foot. This requires a ratio of 20 to 1 which is unacceptable for state of the art plastic processing equipment.
Drying the waste material has also proven economically prohibitive. Its low bulk density limits the amount of material (by weight) that can be charged to a drying machine. The high water content requires long drying cycles. Thus, the combination of a long drying time with a sample load weight results in a very uneconomical and impractical process. In addition, polyethylene is sensitive to heat and will melt or fuse if heated much about 175.degree. or 180.degree. F. Thus, the drying process cannot be accelerated by raising the temperature. Furthermore, any local overheating of the dryer will cause the polyethylene to fuse and stick to the body of the drier. SUMMARY OF THE INVENTION
One object of this invention is to provide a process that will efficiently and economically convert wet, fiber contaminated, low bulk density polyethylene waste material, obtained from hydropulping scrap polyethylene coated paper products, into a dry granulated or pelletized form that can be utilized in standard plastic fabricating machines, such as injection molding machines or extruders.
Another object of this invention is to provide a process that will upgrade the properties and the value of waste polyethylene material by removing the contaminents from the polyethylene material.
It is yet another object of the present invention to provide a novel drying process for economically drying the polyethylene waste material or the defiberized polyethylene waste material that substantially obviates fusing of the waste material to the drier.
Briefly, in accordance with the invention, there is provided a method for converting wet, fiber contaminated, low bulk density, polyethylene waste material into a dry pelletized form. The raw waste material is preferably defiberized, dried in a novel manner, and pelletized into the finished product. Drying of the waste material, which is suitably confined, is by heated air, the temperature of which is controlled at specified location.
Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings.